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, 274 (1629), 3141-9

Effects of Host Migration, Diversity and Aquaculture on Sea Lice Threats to Pacific Salmon Populations

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Effects of Host Migration, Diversity and Aquaculture on Sea Lice Threats to Pacific Salmon Populations

Martin Krkosek et al. Proc Biol Sci.

Abstract

Animal migrations can affect disease dynamics. One consequence of migration common to marine fish and invertebrates is migratory allopatry-a period of spatial separation between adult and juvenile hosts, which is caused by host migration and which prevents parasite transmission from adult to juvenile hosts. We studied this characteristic for sea lice (Lepeophtheirus salmonis and Caligus clemensi) and pink salmon (Oncorhynchus gorbuscha) from one of the Canada's largest salmon stocks. Migratory allopatry protects juvenile salmon from L. salmonis for two to three months of early marine life (2-3% prevalence). In contrast, host diversity facilitates access for C. clemensi to juvenile salmon (8-20% prevalence) but infections appear ephemeral. Aquaculture can augment host abundance and diversity and increase parasite exposure of wild juvenile fish. An empirically parametrized model shows high sensitivity of salmon populations to increased L. salmonis exposure, predicting population collapse at one to five motile L. salmonis per juvenile pink salmon. These results characterize parasite threats of salmon aquaculture to wild salmon populations and show how host migration and diversity are important factors affecting parasite transmission in the oceans.

Figures

Figure 1
Figure 1
Study area and sample locations for 2005. The spatial distribution of sample sites was similar in 2004 and 2006. Grey dots, dip net sites; black dots, trawl sites. Chatham Sound and Skeena River are located on the north coast of British Columbia, Canada. There are no salmon farms in this area.
Figure 2
Figure 2
Sea lice abundance for all developmental stages combined (±95% bootstrap CIs) in (a) April, (b) May, (c) June and (d) July for L. salmonis (light grey bars) and C. clemensi (dark grey bars) over 3 years (2004–2006).
Figure 3
Figure 3
Effects of increasing motile sea lice infection, P¯, of juvenile pink salmon on juvenile salmon survival from parasites (φ), salmon net reproductive value (R0) and equilibrium abundance relative to abundance at natural sea lice levels for (a) one-month exposure, (b) two-month exposure and (c) three-month exposure to the parasites. Model predictions are bounded by compensatory (right boundary, equation (5.3)) and non-compensatory (left boundary, equation (5.2)) parasite-induced host mortality. The horizontal dotted line shows R0=1, above which salmon populations persist and below which salmon populations collapse.

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